Hard disk substrates are manufactured by a method including electroless NiP plating applied to a machined aluminum or aluminum alloy substrate to form a plating film as a base of a magnetic film (see Patent Document 1).
Herein, for higher-density recording by hard disk devices, it is required to minimize the floating height of a recording/reproducing head from the surface of a hard disk substrate. To this end, following the formation of a plating film by electroless NiP plating, polishing is performed to smooth the surface of the plating film using free abrasive grains.
Patent Document 1: JP Patent Publication (Kokai) No. 03-236476 A (1991)
Since the surface of the plating film by electroless NiP plating is very rough, heave burden is imposed on the polishing process. Further, since the removal thickness with polishing is large, the plating film also has to be made thick. Therefore, productivity deteriorates and environmental burden increases.
From such a viewpoint, it has been desired to smooth the surface of a plating film formed by electroless NiP plating as flat as possible to reduce the burden on the polishing process. For instance, when a plating film is formed on a printed board or the like, a brighter such as an organic sulfur compound is added to electroless plating bath, thus forming a plating film having a smooth surface.
In general, however, a plating film containing sulfur has poor corrosion resistance against the acid solution, and especially during the manufacturing process for hard disk substrates using strong-acid slurry in the polishing step, defects such as corrosion pits may occur on the surface of the plating film, and therefore techniques of printed boards cannot be applied directly. Further, such poor corrosion resistance against the acid solution of the plating film may cause excessive elution of Ni preferentially from the plating film during washing with strong acid, which may lead to problems in the subsequent steps for the hard disk substrates.
In view of the above issues, it is an object of the invention to provide a method for production of a hard disk substrate capable of obtaining a smooth surface of a plating film by electroless NiP plating that is not degraded in corrosion resistance against the acid solution and such a hard disk substrate.